Customary method steps in the industrial use of reusable metal containers are cleaning and sterilization with superheated steam, and filling, temperature-adjustment, transport and emptying of liquids. With the exception of the cleaning step, the processes mentioned require a sterile-filtration venting element (venting device) on at least one container opening (flange) in order to prevent equipment damage owing to positive or negative pressure and, at the same time, to ensure the absence of microbes in the solution-contacted interior during venting.
The venting element is the interface between a preferably sterile, liquid-containing container interior (for example, in the form of a liquid barrier in dialysis devices, infusion solution containers or in fermenters) and an exterior, preferably nonsterile atmosphere. In most cases, a sterile-filtration membrane filter composed of a synthetic polymer is selected as the actual separation medium in the venting element. In rare cases, a nonwoven composed of synthetic fiber material is incorporated.
In many cases, synthetic polymers have hydrophobic surface properties which are attributed to the intrinsic hydrophobicity of the synthetic materials. The hydrophobicity is a material constant. It is caused by the extramolecular interactions of the atom groups forming the polymer.
Owing to their low surface tension with respect to water, these materials have reduced wettability with aqueous and polar media. For smooth, nonporous surfaces, the contact angle with respect to water is a measure of the surface tension. Surfaces with a contact angle of more than 90° with respect to water are referred to as hydrophobic. Hydrophobic substances are not miscible or wettable with water. The substances are usually nonpolar and their surface tension is below 72 mN/m at 20° C. Oleophobic substances, which have an especially high hydrophobicity, are not miscible or wettable with oils and other nonpolar substances. Their surface tension is less than 21 mN/m at 20° C. Typical surface tensions of polymers which are processed to form membranes and their contact angles with respect to water are listed in table 1.
TABLE 1Surface tensions of smooth, nonporous polymersand their contact angles with respect to waterContact angle withSurface tensionrespect to waterPolymer[mN/m][°]Polyamide (nylon)75a49aPolyethersulfone (PES)58a54aPolyetheretherketone (PEEK)49a71aPolyethylene (PE)31b94bPolyvinylidene fluoride (PVDF)25b85bPolytetrafluoroethylene (PTFE)  18.5b108b aMembrane Science and Technology Series, 11, “Membrane Contactors: Fundamentals, Applications and Potentialities”, 2005, E. Drioli et al.bJ. Appl. Polym. Sci., 1969, 13, 1741-1747, D. K. Owens et al.
The hydrophobic character of the sterile-filtration separation medium is a prerequisite for incorporation into a venting element for two different reasons. Firstly, no closed water film must form on the surface of or within the separation medium upon contact with water or aqueous medium or, in particular, steam (during steaming or gassing of bioreactors). The water film would prevent the pressure exchange (gas exchange) between the inner and outer atmosphere and thereby compromise the mechanical integrity of the container. In this case, a strong hydrophobicity (e.g., as in the case of fluorine-containing organic polymers) through to the oleophobic character of the separation medium is advantageous.
For instance, venting applications make use of customary materials for membrane filters, such as polytetrafluoroethylene (PTFE), polypropylene (PP) and polyvinylidene fluoride (PVDF), and polyethylene (PE) is used in the case of fiber material.
As is evident from table 1, perfluorinated materials, such as polytetrafluoroethylene (PTFE) for example, exhibit especially hydrophobic properties. If the starting polymer does not contain any fluorine substituents, as is the case for example for polysulfone (PSU) or polyethersulfone (PES), a modification of the membrane surface with fluorine-containing agents in monomeric, oligomeric or polymeric form is possible in order to lower the surface tension of the polymer, and so wetting with liquids of low surface tension, such as surfactant solutions, alcohols or oils for example (cf. table 2), does not occur.
TABLE 2Surface tension of liquidsLiquidSurface tension [mN/m]Water 72.88aParaffin23b  Ethanol22.3cMethanol22.5cn-Octane21.8aaA. W. Adamson, Physical Chemistry of Surfaces, 6th ed., Wiley 1997bJ. Appl. Polym. Sci., 1969, 13, 1741-1747, D. K. Owens et al.cJ. Chem. Eng. Data, 1981, 26, 323-333, G. Körösi et al.
In the prior art, various methods for providing membranes having both hydrophobic and oleophobic properties have been described.
For instance, U.S. Pat. No. 5,217,802 and U.S. Pat. No. 5,286,382 describe porous membranes having a polymer coating which originates from the in situ crosslinking of polymers produced from monomers having fluorine substituents. The monomers preferably used are fluoroalkene, fluoroacrylate or fluorostyrene derivatives or fluoroalkylsiloxanes. The membranes provided with the polymer coating have a surface tension of more than 21 dynes/cm (21 mN/m).
WO 2009/065092 A1 discloses microporous textile-reinforced polyolefin membranes composed of PE, the main surfaces of which are rendered selectively hydrophobic and oleophobic, i.e., having surface tensions of less than 21 mN/m, by an impregnation method. By means of the aforementioned impregnation method, it is possible for one main surface of the microporous PE membrane to be made oleophobic with a fluorine substituent-containing polymer, whereas the opposite main surface of the PE membrane retains its hydrophobic starting properties. A disadvantage of these membranes known from WO 2009/065092 A1, which have been proven effective in principle as breathable materials in clothing manufacture, is that they do not exhibit sufficient resistance with respect to high-energy radiation, for example gamma radiation, and have only inadequate temperature stability.
U.S. Pat. No. 6,579,342 B2 describes the production of an oleophobic venting filter for fluids to be administered intravenously. The venting filter is produced by grafting a fluorosulfone oligomer having perfluorinated alkylsulfonamide groups onto a polymeric substrate. The polymeric substrate involves preferably poly(ether)sulfones, polyamide, PVDF, polyacrylates or PTFE.
Such filter membranes from the prior art are characterized by a distinctly lower surface tension than the nonwetting medium, caused by the chemical properties of the membrane surface.
A self-cleaning effect with water or aqueous media is achieved on intrinsically hydrophobic materials, for example polymers as listed in table 1. This effect is used technically to obtain self-cleaning materials, since dirt particles on such coatings have only few separate contact points with the surface and can therefore be easily rinsed off.
This drip-off effect, the so-called “lotus effect”, is familiar to nonporous surfaces such as films, textile fibers or metal parts and is, for example, achieved by imprinting and impressing surface structures or by partly removable application of particulate coatings. This lotus effect which is used technically is modelled on the self-cleaning effect observed in lotus plants. In lotus plants, this self-cleaning effect is caused by a hydrophobic double structure of the surface, whereby the contact area and thus the adhesion force between the surface and the overlying particles and water drops is greatly reduced to such an extent that self-cleaning occurs. This double structure is the result of a characteristically formed epidermis of the lotus plants, with waxes being situated on the outermost epidermal layer. These supported waxes are hydrophobic and form the second part of the double structure. Thus, it is no longer possible for water to reach the interspaces of the leaf surface, and so the contact area between surface and water is drastically reduced.
EP 2 011 629 A1 discloses microarrays which have a polymer coating and on which surface regions can be selectively roughened and hydrophobicized by laser irradiation in order to produce a lotus effect. The laser irradiation is preferably carried out at an energy density which only leads to roughening of the surface but not to removal of polymer material from the irradiated surface, i.e., the energy density is below the ablation limit.